The storage system includes a host and a storage apparatus for storing data of the host. Recently, intensified business activity demands increase in the capacity of data to be stored in the storage apparatus. Accordingly loss of those data may give a serious influence on such business activity.
In order to secure the data stored in the storage apparatus, data duplication has been conducted in the storage apparatus. The aforementioned method is very effective for coping with the disk failure and the hardware failure in the storage apparatus. In the case where the apparatus itself is damaged by such disaster as earthquake, the method may fail to secure the data which have been duplicated in the storage apparatus. The storage apparatus is duplicated as the countermeasure against the natural disaster in various forms, for example, the duplicated storage apparatuses are installed on different floors, or in different cities.
The storage system is provided with a remote copy function for copying the data among the aforementioned storage apparatuses. The storage system uses the remote copy function between remotely located storage apparatuses, which allows mirroring of the database as the countermeasure against the natural disaster. Generally the database system includes means for recovering data in case of the failure of the apparatus such as the server and the natural disaster by conducting self control/assurance of the sequence for writing to the data storage apparatus such as the storage apparatus. The storage apparatus which uses the remote copy function is required to reflect data on the mirroring storage apparatus while maintaining the sequence of the data written from the host. Irrespective of the failure in the host during the operation, the data processing may be continued based on the data stored in the mirrored storage apparatus using the auxiliary storage system host so long as the sequence is secured.
FIG. 1A illustrates the remote copy control method of the storage apparatus as related art. The mirroring is executed by transferring data between a buffer 33 exclusive for a control module 35 (CM0, CM1) of the storage apparatus 31 as the copy source, and a buffer 43 exclusive for a control module 45 (CM0, CM1) of a remotely connected storage apparatus 41.    1) The storage apparatus 31 as the copy source temporarily stores data I/O (1)-I/O (5) from the host in a disk unit 34, and thereafter, stores the data in the buffer 33 for the transfer.    2) Periodical data transfer is performed between the buffers 33 and 43 from the storage apparatus 31 as the copy source to the storage apparatus 41 as the copy destination. In the storage apparatus 31 as the copy source, the transfer timing from the buffer 33 to the storage apparatus 41 as the copy destination is synchronized between the CM0-CM1.    3) The storage apparatus 41 as the copy destination reflects the data transferred from the storage apparatus 31 as the copy source on the disk unit 44. In the storage apparatus 41 as the copy destination, the output timing of the buffer 43 is synchronized between CM0-CM1.
Plural buffers 33 and 43 are employed, and data are transferred from the storage apparatus 31 as the copy source to the storage apparatus 41 as the copy destination while maintaining each sequence of the plural buffers 33 and 43, respectively and correlating the buffers 33 and 43. As a result, the sequence of data in the storage apparatus 41 as the copy destination is secured. Recently, the data assurance has been regarded as being more important for the system. Not only the single apparatus but also the plural apparatuses are required to be mirrored. In the case where the remote copy to the plural apparatuses is supported as the extension of the generally employed technology, the following problems may occur.
FIG. 1B illustrates a storage apparatus 51 at the intermediate location.    1) The data transferred from the storage apparatus 31 as the copy source, and stored in a buffer 53-1 are temporarily stored in a disk unit 54. The output of the data in the buffer 53-1 to the disk units 54 is conducted in parallel without considering the data sequence.    2) The data stored in the disk unit 54 are stored in a buffer 53-2, and then transferred to the storage apparatus 41 as the copy destination. The data are stored in the buffer 53-2 in the sequence of writing to the disk unit 54, and accordingly, the data are transferred to the storage apparatus 41 without securing the sequence as described above. The storage apparatus 51 requires double buffers, that is, buffers 53-1 and 53-2. The remote copy between the storage apparatuses 51 and 31, and the remote copy between the storage apparatuses 51 and 41 are separately controlled. So the data sequence cannot be secured among the plural apparatuses (refer to Japanese Laid-open Publication No. 2006-260292).